文摘
Accurate 182Hf–182W chronology of early planetary differentiation relies on highly precise and accurate tungsten isotope measurements. WO3– analysis by negative thermal ionization mass spectrometry requires W17O16O2–, W17O216O–, W18O16O2–, W17O3–, W17O18O16O–, and W18O216O– isotopologue interference corrections on W16O3– species (Harper et al. Geochim. Cosmochim. Actama-instance" xml:space="preserve"> 1996ma-instance" xml:space="preserve">, 60ma-instance" xml:space="preserve">, 1131; Quitté et al. Geostandard. Newslett.ma-instance" xml:space="preserve"> 2002ma-instance" xml:space="preserve">, 26ma-instance" xml:space="preserve">, 149; Trinquier et al. Anal. Chem.ma-instance" xml:space="preserve"> 2016ma-instance" xml:space="preserve">, 88ma-instance" xml:space="preserve">, 1542; Touboul et al. Naturema-instance" xml:space="preserve"> 2015ma-instance" xml:space="preserve">, 520ma-instance" xml:space="preserve">, 530; Touboul et al. Int. J. Mass Spectrom.ma-instance" xml:space="preserve"> 2012ma-instance" xml:space="preserve">, 309ma-instance" xml:space="preserve">, 109). In addition, low ion beam intensity counting statistics combined with Faraday cup detection noise limit the precision on the determination of 18O/16O and 17O/16O relative abundances. Mass dependent variability of 18O/16O over the course of an analysis and between different analyses calls for oxide interference correction on a per integration basis, based on the in-run monitoring of the 18O/16O ratio (Harper et al. Geochim. Cosmochim. Actama-instance" xml:space="preserve"> 1996ma-instance" xml:space="preserve">, 60ma-instance" xml:space="preserve">, 1131; Quitté et al. Geostandard. Newslett.ma-instance" xml:space="preserve"> 2002ma-instance" xml:space="preserve">, 26ma-instance" xml:space="preserve">, 149; Trinquier et al. Anal. Chem.ma-instance" xml:space="preserve"> 2016ma-instance" xml:space="preserve">, 88ma-instance" xml:space="preserve">, 1542). Yet, the 17O/16O variation is normally not being monitored and, instead, inferred from the measured 18O/16O variation, assuming a δ17O−δ18O Terrestrial Fractionation Line (Trinquier et al. Anal. Chem.ma-instance" xml:space="preserve"> 2016ma-instance" xml:space="preserve">, 88ma-instance" xml:space="preserve">, 1542). The purpose of the present study is to verify the validity of this assumption. Using high resistivity amplifiers, 238U17O2 and 238U18O2 ion beams down to 1.6 fA have been monitored simultaneously with 235,238U16O2 species in a uranium certified reference material. This leads to a characterization of O isotope fractionation by thermal ionization mass spectrometry in variable loading and running conditions (additive-to-sample ratio, PO2 pressure, presence of ionized metal and oxide species). Proper determination of O isotope composition based on the simultaneous analysis of the 18O/16O and 17O/16O ratios could prevent tens of ppm bias or more on the 182W/184W and 183W/184W ratios.